The manufacture of a variety of products having high quality, particularly in the fields of electric appliances for domestic use, and machines or furniture for office use, has created a growing demand for weldable and colored steel sheets having an excellent appearance which are made by the continuous and rapid treatment of steel sheets plated with zinc or an alloy thereof. The demand extends to sheets having a variety of colors.
There are known a number of methods which enable the manufacture of weldable black steel sheets in a continuous strip line. It has, however, been difficult to make a weldable steel sheet having any color other than black, since it has been difficult to produce a uniform color in a wide surface area.
The following is a brief description of the known methods for forming a colored film on a steel sheet, or a steel sheet plated with zinc:
(a) A resin solution containing an organic or inorganic pigment is applied onto a steel sheet by e.g. spraying or roll coating to form thereon a film having a thickness of several tens of microns. PA1 (b) A plating layer existing on a steel sheet is caused to undergo reaction or electrolysis to develop a colored film. This method includes variations as will hereunder be set forth: PA1 (c) The surface of a steel sheet, or a steel sheet plated with zinc is subjected to cathode electrolysis treatment to form a colored film (e.g. Japanese Patent Application Laid-Open No. 10292/1987). PA1 (d) A solution obtained by adding an organic dye to an aqueous solution of potassium sodium silicate is applied onto a steel surface plated with zinc or an alloy thereof to improve corrosion resistance and adhesion (Japanese Patent Publication No. 30593/1980). PA1 (a) This is a common method of coating which employs an organic or inorganic pigment as a coloring agent. The uniform distribution of the pigment in a film is, however, essential to ensure a uniform color tone, and calls for a film thickness of at least about 10 microns. This thickness is too large to permit spot welding. The maximum film thickness that permits welding has been about three microns, and has been too small to ensure the uniform distribution of the pigment. PA1 (b) All of the methods as described at (b-2), and (b-5) to (b-7) are useful for forming only a black film. PA1 (c) This method is mainly employed for forming a black film and can only form a film of poor adhesion. PA1 (d) This method is not intended for forming a film having an excellent appearance, nor is it intended for imparting weldability, insofar as the disclosure does not contain any specific reference to the film thickness. Moreover, as the film is basically composed of potassium sodium silicate, the hardened film is unsatisfactory in lubricating property as required during the press forming of the steel sheet, and is, therefore, unsuitable for any steel sheet used for making electric appliances for domestic use, office machines or furniture, etc. Moreover, the disclosure does not specifically define the organic dye to be used. There are a very large number of kinds of organic dyes which are widely different from one another in properties including light fastness, tinting strength and solubility in a solvent. Some dyes are superior to others in e.g. tinting strength, but inferior in light fastness. It is, therefore, needless to say that all of the organic dyes are not equally useful. PA1 (1) A weldable colored steel sheet having a surface plated with zinc or a zinc alloy, and carrying a chromate film formed on the plated surface and having a coating weight of 1 to 200 mg/m.sup.2 in terms of metallic chromium, and a colored film formed on the chromate film from a composition comprising 100 parts by weight of a thermosetting resin as a base resin and 1 to 200 parts by weight of a dye as a coloring agent, the resin and the dye being both soluble in an organic solvent, the colored film having a thickness of 0.3 to 3.0 microns, and a color other than black. PA1 (2) A weldable colored steel sheet having a surface plated with zinc or a zinc alloy, and carrying a chromate film formed on the plated surface and having a coating weight of 1 to 200 mg/m.sup.2 in terms of metallic chromium, and a colored film formed on the chromate film from a composition comprising 100 parts by weight of a thermosetting resin as a base resin, 1 to 200 parts by weight of a dye as a coloring agent and 1 to 100 parts by weight of a solid lubricant, the resin and the dye being both soluble in an organic solvent, the colored film having a thickness of 0.3 to 3.0 microns, and a color other than black. PA1 (3) A weldable colored steel sheet having a surface plated with zinc or a zinc alloy, and carrying a chromate film formed on the plated surface and having a coating weight of 1 to 200 mg/m.sup.2 in terms of metallic chromium, and a colored film formed on the chromate film from a composition comprising 100 parts by weight of a thermosetting resin as a base resin, 1 to 200 parts by weight of a dye as a coloring agent and 1 to 100 parts by weight of a granular rust-inhibitive pigment, the resin and the dye being both soluble in an organic solvent, the colored film having a thickness of 0.3 to 3.0 microns, and a color other than black. PA1 (4) A weldable colored steel sheet having a surface plated with zinc or a zinc alloy, and carrying a chromate film formed on the plated surface and having a coating weight of 1 to 200 mg/m.sup.2 in terms of metallic chromium, and a colored film formed on the chromate film from a composition comprising 100 parts by weight of a thermosetting resin as a base resin, 1 to 200 parts by weight of a dye as a coloring agent, 1 to 100 parts by weight of a solid lubricant and 1 to 100 parts by weight of a granular rust-inhibitive pigment, the resin and the dye being both soluble in an organic solvent, the colored film having a thickness of 0.3 to 3.0 microns, and a color other than black.
(b-1) A steel sheet is dipped in a chromating solution containing chromic acid and another acid, and if the bath composition and the reaction temperature are appropriately varied, it is possible to form a lustrous rainbow-colored film, an unlustrous yellowish green film, or a differently colored film on a steel surface plated with pure zinc; PA2 (b-2) A chromating solution containing Ag.sup.+ ions is used to form a black chromate film on a steel surface plated with pure zinc (as described in Japanese Patent Application Laid-Open No. 193376/1983); PA2 (b-3) A chromate film is formed on a steel surface plated with pure zinc, and the steel surface is, then, dyed by dipping in a dye solution; PA2 (b-4) A steel surface plated with pure zinc is dipped in a solution containing Ni.sup.2+ or Cu.sup.2+ ions, so that the metal which the solution contains may be deposited on the steel surface to form a colored film thereon; PA2 (b-5) A plating layer of pure zinc is subjected to anodic oxidation in an alkali solution to form a black film; PA2 (b-6) An electroplating layer of a Zn--Co, Zn--Ni or Zn--Mo alloy is subjected to anodic treatment to form a black film (Japanese Patent Publication No. 38276/1986); and PA2 (b-7) A steel sheet which has been plated with a Zn--Ni alloy is subjected to dipping, spraying, or anodic treatment with a solution containing nitric acid or a nitrate group to develop a black surface (Japanese Patent Publication No. 30262/1987). PA2 (b-1) This method is likely to form an unevenly colored surface, since its color is greatly affected by even a small difference in thickness of a chromate film. Moreover, it can form only a film having a yellowish or greenish color which is specific to a reactive chromate. PA2 (b-2) This method is useful for forming only a black film, as hereinabove stated. Moreover, the solution containing Ag.sup.+ ions is very expensive. PA2 (b-3) The dyeing of the chromate film requires a period of several or more minutes. This period is too long for the continuous coloring treatment of a steel strip, for which only a period not exceeding five seconds is permissible. PA2 (b-4) This method requires a treating time of as long as several or more minutes, but can form only a film of poor adhesion. Moreover, a film formed by the deposition of Cu.sup.2+ ions has the drawback of changing in color as a result of oxidation with the passage of time. PA2 (b-5) This method is also unsuitable for the continuous treatment of a strip, since it requires a treating time which is as long as 5 to 20 minutes. PA2 (b-6) and (b-7) These methods are both useful for forming only a black film, as hereinabove stated, though they can be carried out rapidly. Moreover, the dissolution of metal ions from a plating layer is not only uneconomical, but also obstructs continuous operation seriously, as the dissolved metal ions deteriorate the solution for the blackening treatment. Furthermore, both of the methods have only a limited scope of application, i.e. (b-6) is applicable only to a steel sheet plated with a Zn--Co, Zn--Ni or Zn--Mo alloy, while (b-7) is applicable only to a sheet plated with a Zn--Ni alloy.
All of these methods do, however, have their own drawbacks, as will hereunder be pointed out.